An image band pass filter operating in a photoelectron spectromicroscope is known and comprises two stages, the first comprising a region in which a beam of imaging electrons is deflected in the crossed electrostatic and magnetic field between a succession of electron mirrors which are designed to remove electrons of a certain energy and reflect those of a particular energy which are to form the electron image.
The beam from the first stage is then transferred to the second stage through a high pass filter into a region of the second stage and then deflected in an electrostatic and magnetic cross field along a path corresponding to a restoration of the displacement of the original beam in the first stage from its original path of travel.
In the first and second stage of the analyser as above described, the electrostatic field of the cross field is designed to be non-uniform whereby to control motion of the electrons in the cross field and correct for distortion of the image in the output beam.
In such spectroscopy the signal to background ratio is of high significance particularly in the detection of weak signals. The first low pass filter encountered by the image stream in the band pass analyser as described above, plays a vital role in determining the signal/scattered electron background ratio. This arises from the unwanted higher energy electrons obtaining axial momentum in the band pass range after elastic and inelastic collision with the absorber plate of the low pass filter and hence being transported through the analyser.
Such scattered electrons may have both large energy and low axial momentum thus contributing a diffuse background within the pass band of the subsequent low pass and high pass elements of the analyser.